1. Field of the Invention
The present invention relates to a printing method for reducing the problems occurring when printing a high-resolution image using a resolution expanding technique or the like, and to a printing apparatus using such the printing method.
2. Description of the Related Art
In recent years, various OA (office automation) devices such as personal computers, word processors, and personal information terminal devices have become very popular. A printing apparatus, usually called a printer, is widely used as a device for outputting information, given from such an OA device, onto a printing medium. With the increasing popularity of OA systems and devices, multimedia has become popular in various applications. As a result, it is often required that computers should deal with information including not only simple characters but also full color images. Such the changes in the environment surrounding the information systems and devices have increased the requirements for high operating speed and high image quality in printers.
One known technique for increasing the operation speed and the image quality of printers is to employ a multi-element print head on which a plurality of printing elements are disposed. It is known to increase the efficiency of driving the multi-element print head by grouping the printing elements into blocks.
The block driving technique will be described in further detail below referring to FIG. 1.
In FIG. 1, reference symbols n1, n2, . . . , n16 denote relative positions of printing elements of a print head. In the case of a printer of the bubble-jet type (hereafter also referred to as a BJ printer) in which ink is heated by a heating element so that a bubble is generated in the ink thereby emitting an ink droplet, nozzles for emitting ink are disposed at positions denoted by these symbols n1 to n16. The nozzles n1 to n16 are aligned in a straight line so that they form a series of nozzles. A printing operation is performed while moving the series of nozzles in a horizontal direction shown in FIG. 1. Therefore, the positions n1 to n16 shown in FIG. 1 also correspond to the positions at which image elements are formed by ink droplets emitted by the nozzles.
In FIG. 1, a plurality of series of dots shown on the right side denote relative positions at which the nozzles are located as the print head travels, wherein reference numerals 1 to 16 shown at the top of the figure denote the positions of the series of nozzles.
Although the number of nozzles disposed on one head usually ranges from a few tens to a few hundreds, it is assumed here for convenience of explanation that the ink-jet print head has 16 nozzles.
In FIG. 1, the distance between adjacent vertical lines at equal intervals denotes the pitch of image dot elements. In this example, the printer is assumed to have a resolution of 360 dots per inch and thus have a dot pitch of about 71 .mu.m. In the present example, the print head is mounted on a printer so that the series of dot elements of the print head is slanted by certain degrees from the direction perpendicular to the scanning direction of the print head so that dots are printed exactly along a vertical line when the printing elements of the print head are driven in a time division fashion. In the time division driving technique, a plurality of printing elements are grouped into some blocks so that each block consists of a plurality of printing elements, and the printing operation is performed block by block at predetermined time intervals. Therefore, in the printing operation according to the time division driving technique, a driving signal is not applied to all printing elements at the same time. This prevents the driving voltage from dropping down to a level smaller than a lower limit, and also prevents the nozzles of the ink-jet printer from becoming short of ink, which would otherwise occur when ink was emitted from a great number of nozzles at the same time. If the time division driving technique is applied to a print head on which all printing elements or blocks are disposed in a line exactly oriented along the vertical direction, the printed image will be slanted by an amount corresponding to the time difference in the driving operation. To avoid the above problem, the orientation of the printing elements on the print head is slanted by an amount corresponding to the time difference in the driving operation so that each printing element emits an ink droplet at a position exactly located on a vertical line.
As shown in FIG. 1, the orientation of the series of nozzles on the print head is slanted so that the nozzle 5 (n5) is located at a position preceding the nozzle 1 (n1) wherein the distance measured in the horizontal direction between the nozzle 5 (n5) and the nozzle 1 (n1) is equal to the resolution pitch (the minimum dot-to-dot distance). While traveling over a printing medium, the above-described print head forms dots on the printing medium according to the print data thereby forming an image on the printing medium. For example, in the case where one line of dots is printed along a column denoted by the arrow in FIG. 1, the nozzle 1 is driven when the print head comes to a position denoted by reference numeral 1 in FIG. 1, and the nozzle 2 is driven when the print head comes to a position 2. The remaining nozzles 3 to 16 are driven in a similar manner thereby forming dots along a vertical line as shown in FIG. 1.
In the print head described above, the nozzle 1 (n1), nozzle 5 (n5), nozzle 9 (n9), and nozzle 13 (n13) are located so that they are apart from each other by an amount corresponding to one column and thus they are driven at the same time. This means these nozzles belong to the same block. Similarly, the nozzle 2 (n2), nozzle 6 (n6), nozzle 10 (n10), and nozzle 14 (n14) are grouped into another block, the nozzle 3 (n3), nozzle 7 (n7), nozzle 11 (n11), and nozzle 15 (n15) are grouped into still another block, and the nozzle 4 (n4), nozzle 8 (n8), nozzle 12 (n12), and nozzle 16 (n16) are grouped into the final block. In this example, the maximum number of nozzles which are driven at the same time is four, and a greater number of nozzles are never driven at the same time. In contrast, in the case of print heads which are not based on the time division technique, there is a possibility that all sixteen nozzles disposed on a print head are driven at the same time. Therefore, the time division driving technique leads to a great reduction in the capacity of the power source and thus a reduction in cost. Furthermore, in the present example, since printing elements are disposed across a plurality of columns (printing positions corresponding to the resolution pitch), it is easier to accurately control the target position at which an ink droplet arrives than in the case in which one column is printed during one driving cycle using a print head on which nozzles are disposed along a vertical line. Thus, a straight line along a column can be formed by controlling the driving timing of the printing elements present on the column and also by controlling the movement of the printing elements. This means that the slanted-nozzle print head offers a high-quality image.
One known technique for achieving high quality in a printed image is to form dots by driving each printing element in a PWM fashion using a multi-division driving pulse such as that shown in FIG. 2, in which the pulse width of the driving pulse is modulated according to the status of the print head. In the example of the multi-division pulse shown in FIG. 2, after a setup period P0, a pre-pulse P1 is applied so as to generate thermal energy within the range which does not lead to emission of ink. Following the pre-pulse P1, the pulse is turned off during a period P2 (off-period). After that, a main pulse P3 is applied thereby emitting an ink droplet. In the PWM driving technique, various parameters may be modulated for achieving the purpose. One way is to modulate the pre-pulse P1. Another way is to modulate the off-time P2 thereby control the time period during which the thermal energy given by the pre-pulse P1 diffuses over the ink. Otherwise, the main pulse P3 may be modulated so as to control the thermal energy for emitting the ink droplet. Either any single of these parameters or any combination of these parameters may be employed.
In the conventional techniques described above, however, there are conflicts between the techniques for achieving the high operating speed and those for achieving the high quality image.
For example, if the number of printing elements is increased twice so as to increase the printing speed twice, then it is required to increase the number of blocks since there is a limitation in the maximum number of nozzles which can be driven at the same time. If the driving frequency is set to 6 kHz, and if the number of blocks is 8, then a driving period of about 20 .mu.s can be assigned to each block. However, if there are 16 blocks, only 10 .mu.s can be assigned to a driving pulse for each block. On the other hand, if the driving frequency is increased twice so as to increase the operating speed, then the driving period becomes half the original period, and thus similar problems occur.
On the other hand, to improve image quality using the PWM technique, it is desirable that the time period assigned for emission of each ink droplet should be as long as possible so that the width of each driving pulse can be long enough. However, this requirement conflicts with the high speed requirement.
It is known in the recent art to achieve a resolution higher than that corresponding to the pitch of printing elements disposed on a print head thereby obtaining high image quality. Also in this technique of expanding the resolution, however, similar problems occur. For example, if the resolution is expanded from 360 dots per inch to 720 dots per inch, it is required to print twice the number of columns over the same printing range. As a result, the pulse width allowed to be assigned to each block decreases to half the original value. On the other hand, it is expected that multi-level printing techniques for modulating the dot size so as to obtain gradation will be important to achieve higher image quality. Also in the multi-level printing techniques, it is desirable that the driving pulse width should be as long as possible.
The technique for improving the image quality is not limited to that in which the pulse width for driving the print head is controlled, and there can be various other techniques for the same purpose. In any case, it is desirable to achieve both a high operating speed and high image quality. This is also true from the viewpoint of stable operation of the print head and the viewpoint of directly controlling the operation of the print head.
In one known technique to achieve both the high operating speed and the high image quality, the number of nozzles which are driven at the same time is increased while nozzles are grouped into blocks in such a manner that a plurality of successive nozzles belong to the same block thereby achieving high image quality in particular associated with the linearity along a column. An example of such a technique will be described in detail below referring to FIG. 3 in which the printing density along the scanning direction of the print head is expanded twice. Reference symbols and numerals are similar to those used in FIG. 1.
In the case where one line of dots is formed along a column denoted by the arrow in FIG. 3, nozzles n1 and n2 are driven when the print head comes to a position 1, and nozzles n3 and n4 are driven when the print head comes to a position 3. Similarly, the remaining couples of nozzles n5, n6, . . . , n16 are driven when the print head comes to positions 5, 7, . . . , 15, respectively so that one line of dots is formed along a vertical line as shown by solid circles in FIG. 3. In this print head, the nozzles n1, n2, n5, n6, n9, n10, n13, and n14 are required to be driven at the same time and thus these nozzles are grouped into the same block. Similarly, the nozzles n3, n4, n7, n8, n11, n12, n15, and n16 are grouped into the other same block so that they are driven at the same time. In this technique, the maximum number of nozzles which are driven at the same time is 8 and a greater number of nozzles are never driven at the same time. Therefore, it is possible to reduce the capacity of the power source and thus reduce the required cost as opposed to the technique in which there is a possibility that all sixteen nozzles are driven at the same time. Although this technique requires the capacity of the power source twice as greater as that of the example shown in FIG. 1, all four dot elements at the upper part of the printing column are formed only by performing printing operations at positions 1 and 3 without having to perform printing operations at positions 2 and 4. In the example shown in FIG. 1, it is required to perform printing four times (when the head comes to the positions 1, 2, 3, and 4) during an operation from a column to an adjacent column. In contrast, in the example shown in FIG. 3, it is required to perform only two printing operations (at the positions 1 and 3). This allows the print head to spend a time twice longer to move from a column to the next column. If the same pulse width is employed in both examples shown in FIG. 1 and 3, the example shown in FIG. 3 can have a driving frequency twice higher than the example shown in FIG. 1. This means that it is possible to increase the number of dots printed per unit time by twice.
In the printing technique shown in FIG. 3, ink droplets are emitted from a plurality of adjacent nozzles at the same time, and therefore the arrival positions of the ink droplets deviate from the vertical printing column by an amount corresponding to the slant of the print head. For example, in the case of a print head having 128 nozzles capable of printing at a resolution of 160 dots per inch by emitting ink droplets through 8 successive nozzles at the same time, ink droplets are emitted every 16 nozzles, and thus the print head is slanted by about 3.5.degree.(=sin.sup.-1 1/16). This angle produces deviations of about 4 .mu.m (=71 .mu.m.times.sin 3.5.degree.) between adjacent nozzles in the direction across the columns. These deviations are so small that they are not perceptible to human eyes. Therefore, this technique can offer good linearity which is one of factors for achieving high image quality.
However, the printing technique described above have various problems arising from the simultaneous emission of ink through adjacent nozzles. One problem is that bubbling occurs in an incorrect manner, which results in generation of a great number of droplets having a diameter much smaller than that of usual droplets (such droplets having unusually small size are referred to as mist).
In particular, such incorrect bubbling tends to occur when data having a high duty ratio is printed successively. If mist is generated, dirty marks which are perceptible appear on a printing medium. This does not meet the requirement for high image quality.
The incorrect bubbling is due to the following causes.
1. Vibrations of the meniscus surface of nozzles PA0 2. Crosstalk between nozzles
After an ink droplet has been emitted once, if another ink droplet is emitted before a nozzle has been refilled completely with ink, the droplet will be emitted when the meniscus surface is at a position outside the nozzle, and thus incorrect bubbling occurs.
If the nozzles are not isolated in a proper fashion, ink emission is influenced by adjacent nozzles. The influence between adjacent nozzles associated with ink emission is here referred to as crosstalk. The influence between adjacent nozzles depends on whether ink is emitted from the adjacent nozzles at the same time or at different times. Incorrect bubbling tends to occur when adjacent nozzles are driven at the same time.
As can be seen from the above discussion, it is important to solve the problems of generation of mist to achieve high image quality.